In recent years, to protect the earth environment from the aspect of improving fuel efficiency especially in automobiles, technologies have been developed such as stop idling, electromotive power steering, electromotive turbocharger, and brake regeneration. Among these technologies, stop idling, electromotive power steering, and electromotive turbocharger, for example, consume a large amount of current on the order of 100 amperes to activate a starter, steering motor, and turbine drive motor, respectively, thus causing a voltage drop in a power source composed of such as a battery and power generator. A large voltage drop prevents a load powered from the power source from operating satisfactorily. Performing brake regeneration generates a regenerative current on the order of a maximum of 100 amperes, depending on the vehicle speed, thereby increasing the voltage of the power source. Directly supplying such voltage causes supply voltage to the load to increase, possibly preventing normal operation.
As a method of eliminating such influence of temporary voltage fluctuation of a power source, the following measure is proposed against a voltage drop, for example, in Patent Document 1. That is, a voltage drop protection circuit is provided between the battery and auxiliaries. The voltage drop protection circuit may use either of the following method. That is, one includes a capacitor as an auxiliary power supply as described in Patent Document 1, and the voltage is compensated by supplying auxiliaries with power from the capacitor when the battery voltage drops. The other does not include an auxiliary power supply, but the voltage is compensated by raising the voltage of the battery to supply to auxiliaries when the battery voltage drops.
In either of the methods, the voltage drop protection circuit requires a DC/DC converter that converts voltage of a capacitor and/or battery to that required for operation of auxiliaries. Here, a description is made for a concrete example of a voltage drop protection circuit including a capacitor as an auxiliary power supply, using FIG. 22.
In FIG. 22, voltage drop protection circuit 1 includes a DC/DC converter circuit configuration, where its input is connected to the output through coil 2 and diode 3. The connecting point of the cathode of diode 3 with the output is connected to the ground through capacitor 4. Further, the connecting points of coil 2 with diode 3 are connected to the ground through transistor 5, and the base of transistor 5 is connected to the output from controller 6. Controller 6 monitors output voltage of voltage drop protection circuit 1 to on-off control transistor 5. Operation of controller 6 is controlled according to an on-off signal from the outside.
In such voltage drop protection circuit 1, transistor 5 is on-off controlled by controller 6 operating according to an operation on-off signal and charges capacitor 4 with voltage increased by means of coil 2. Additionally, controller 6 monitors output voltage of voltage drop protection circuit 1 to maintain the voltage at a predetermined output set value. Consequently, even in a low-voltage state of the battery, the output voltage of voltage drop protection circuit 1 can be controlled so as not to decrease. The above is an example of a power supply unit compensating a voltage drop of a power source.
The example in Patent Document 1 compensates a voltage drop of a battery. Meanwhile, output voltage of voltage drop protection circuit 1 may be suppressed by storing power in brake regeneration to capacitor 4, for example.
It is true that such a conventional power supply unit can supply a load with stable voltage even if voltage of a power source fluctuates. However, the problem lies in the fact that the voltage output set value of controller 6 is constant. That is, a battery or the like as a power source produces voltage fluctuation relatively small (concretely, a range between approximately 12 V and 14 V) over a long duration according to an environment change such as fluctuation in ambient temperature and/or deterioration. For this reason, the output set value of controller 6 is fixed to a value (e.g. 11 V) lower than the lowest value (12 V) in a long-term voltage fluctuation range of the battery in order to output power from the battery preferentially in normal times, during which a temporary voltage fluctuation of the battery does not occur. With this arrangement, when the normal battery voltage is approximately 12 V (i.e. lowest value), power is supplied to the load with the output voltage from capacitor 4 controlled to be 11 V if the battery voltage temporarily decreases due to such as driving the starter. Accordingly, the load can be kept driven without problems because the supply voltage decreases from 12 V to 11 V at lowest from the viewpoint of the load. If the regular battery voltage is 14 V, however, the voltage results in suddenly decreasing from 14 V to 11 V (the gap is as high as 3 V) from the viewpoint of the load because voltage drop protection circuit 1 outputs a constant voltage of 11 V due to a temporary drop of the battery voltage. This can possibly affect operation depending on a load.
When performing regeneration by braking, the voltage of the power source temporarily increases as well, and thus controller 6 operates so as to charge capacitor 4 when a certain predetermined voltage (e.g. 14.5 V) is exceeded. At this moment, if the voltage of such as a battery before regeneration is 14 V, the voltage fluctuation is small, however if the battery voltage is 12 V, the supply voltage to the load suddenly increases to 14.5 V, which can possibly affect the load in the same way as the above.
Patent Document 1: Japanese Patent Unexamined Publication No. 2005-112250